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          <div>
            <h2 class="title"><a id="repapp"></a>Chapter 3. The DB Replication Manager</h2>
          </div>
        </div>
      </div>
      <div class="toc">
        <p>
          <b>Table of Contents</b>
        </p>
        <dl>
          <dt>
            <span class="sect1">
              <a href="repapp.html#rep_init_code">
                Starting and Stopping Replication
            </a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="repapp.html#election_flags">Managing Election Policies</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="repapp.html#thread_count">Selecting the Number of Threads</a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="repmgr_init_example_c.html">Adding the Replication Manager to
                    
                    <span>RepMgr</span>
                    
            </a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="fwrkpermmessage.html">Permanent Message Handling</a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="fwrkpermmessage.html#fmwrkpermpolicy">Identifying Permanent Message Policies</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="fwrkpermmessage.html#fmwrkpermtimeout">Setting the Permanent Message Timeout</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="fwrkpermmessage.html#perm2fmwrkexample">Adding a Permanent Message Policy to 
                            
                            <span>RepMgrGSG</span>
                            
                    </a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="electiontimes.html">Managing Election Times</a>
            </span>
          </dt>
          <dd>
            <dl>
              <dt>
                <span class="sect2">
                  <a href="electiontimes.html#electiontimeout">Managing Election Timeouts</a>
                </span>
              </dt>
              <dt>
                <span class="sect2">
                  <a href="electiontimes.html#electretrytime">Managing Election Retry Times</a>
                </span>
              </dt>
            </dl>
          </dd>
          <dt>
            <span class="sect1">
              <a href="fmwrkconnectretry.html">Managing Connection Retries</a>
            </span>
          </dt>
          <dt>
            <span class="sect1">
              <a href="heartbeats.html">Managing Heartbeats</a>
            </span>
          </dt>
        </dl>
      </div>
      <p>
        The easiest way to add replication to your transactional
        application is to use the Replication Manager. The Replication Manager provides a comprehensive 
        communications layer that enables replication. For a brief listing
        of the Replication Manager's feature set, see 
        <a class="xref" href="apioverview.html#repframeworkoverview" title="Replication Manager Overview">Replication Manager Overview</a>.
    </p>
      <p>
        To use the Replication Manager, you make use of special methods off the
        
        <span><code class="classname">DbEnv</code> class.</span>
        That is: 
    </p>
      <div class="orderedlist">
        <ol type="1">
          <li>
            <p>
                                    Create an environment handle as normal.
                            </p>
          </li>
          <li>
            <p>
                                    Configure your environment handle as
                                    needed (e.g. set the error file and
                                    error prefix values, if desired).
                            </p>
          </li>
          <li>
            <p>
                                    Use the Replication Manager replication methods to
                                    configure the Replication Manager. Using these
                                    methods causes DB to know that you
                                    are using the Replication Manager.
                            </p>
            <p>
                                    Configuring the Replication Manager entails setting the replication
                                    environment's priority, setting the TCP/IP address
                                    that this replication environment will use for
                                    incoming replication messages, identifying
                                    TCP/IP addresses of other replication
                                    environments, setting the number of
                                    replication environments in the
                                    replication group, and so forth. These actions are
                                    discussed throughout the remainder of
                                    this chapter.
                            </p>
          </li>
          <li>
            <p>
                                    Open your environment handle. When you
                                    do this, be sure to specify 

                                    <span><code class="literal">DB_INIT_REP</code> and
                                    <code class="literal">DB_THREAD</code> to your
                                    open flags. (This is in addition to the
                                    flags that you normally use for a
                                    single-threaded transactional
                                    application).  The first of these causes
                                    replication to be initialized for the
                                    application. The second causes your
                                    environment handle to be free-threaded
                                    (thread safe). Both flags are required
                                    for Replication Manager usage.
                                    </span>
                                    
                                    
                            </p>
          </li>
          <li>
            <p>
                                    Start replication by calling
                                    
                                    <span><code class="methodname">DbEnv::repmgr_start()</code>.</span>
                                    
                            </p>
          </li>
          <li>
            <p>
                                    Open your databases as needed. Masters
                                    must open their databases for read
                                    and write activity. Replicas can open
                                    their databases for read-only activity, but
                                    doing so means they must re-open the
                                    databases if the replica ever becomes a
                                    master.  Either way, replicas should never attempt to
                                    write to the database(s) directly.
                            </p>
          </li>
        </ol>
      </div>
      <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
        <h3 class="title">Note</h3>
        <p>
                            The Replication Manager allows you to only use one
                            environment handle per process.
                    </p>
      </div>
      <p>
                    When you are ready to shut down your application: 
            </p>
      <div class="orderedlist">
        <ol type="1">
          <li>
            <p>
                                    Close your databases
                            </p>
          </li>
          <li>
            <p>
                                    Close your environment. This causes
                                    replication to stop as well.
                            </p>
          </li>
        </ol>
      </div>
      <div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
        <h3 class="title">Note</h3>
        <p>
                            Before you can use the Replication Manager, you may have to
                            enable it in your DB library. This is
                            <span class="emphasis"><em>not</em></span> a requirement for
                            Microsoft Windows systems, or Unix systems that
                            use pthread mutexes by default. Other systems,
                            notably BSD and BSD-derived systems (such as
                            Mac OS X), must enable the Replication Manager when you
                            configure the DB build.
                    </p>
        <p>
                            You do this by <span class="emphasis"><em>not</em></span>
                            disabling replication and by configuring the
                            library with POSIX threads support. In other
                            words, replication must be turned on in the
                            build (it is by default), and POSIX thread
                            support must be enabled if it is not already by
                            default.  To do this, use the
                            <code class="literal">--enable-pthread_api</code> switch
                            on the configure script.
                    </p>
        <p>
                        For example:
                    </p>
        <pre class="programlisting">../dist/configure --enable-pthread-api</pre>
      </div>
      <div class="sect1" lang="en" xml:lang="en">
        <div class="titlepage">
          <div>
            <div>
              <h2 class="title" style="clear: both"><a id="rep_init_code"></a>
                Starting and Stopping Replication
            </h2>
            </div>
          </div>
        </div>
        <div class="toc">
          <dl>
            <dt>
              <span class="sect2">
                <a href="repapp.html#election_flags">Managing Election Policies</a>
              </span>
            </dt>
            <dt>
              <span class="sect2">
                <a href="repapp.html#thread_count">Selecting the Number of Threads</a>
              </span>
            </dt>
          </dl>
        </div>
        <p>
                    As described above, you introduce replication to an
                    application by starting with a transactional
                    application, performing some basic replication
                    configuration, and then starting replication using 
                    
                    <span><code class="methodname">DbEnv::repmgr_start()</code>.</span>
                    
            </p>
        <p>
                    You stop replication by closing your environment
                    cleanly in the same way you would for any DB application.
            </p>
        <p>
                For example, the following code fragment initializes, then
                stops and starts replication. Note that other replication
                activities are omitted for brevity.
            </p>
        <pre class="programlisting">#include &lt;db_cxx.h&gt;

/* Use a 10mb cache */
#define CACHESIZE   (10 * 1024 * 1024)

...

    DbEnv *dbenv;            /* Environment handle. */
    const char *progname;     /* Program name. */
    const char *envHome;      /* Environment home directory. */
    const char *listen_host;  /* A TCP/IP hostname. */
    const char *other_host;   /* A TCP/IP hostname. */
    u_int16 listen_port;      /* A TCP/IP port. */
    u_int16 other_port;       /* A TCP/IP port. */

    /* Initialize variables */
    dbenv = NULL;
    progname = "example_replication";
    envHome = "ENVIRONMENT_HOME";
    listen_host = "mymachine.sleepycat.com";
    listen_port = 5001;
    other_host = "anothermachine.sleepycat.com";
    other_port = 4555;

    try {
        /* Create the environment handle */
        dbenv = new DbEnv(0);

        /*
         * Configure the environment handle. Here we configure 
         * asynchronous transactional commits for performance reasons. 
         */
        dbenv-&gt;set_errfile(stderr);
        dbenv-&gt;set_errpfx(progname);
        (void)dbenv-&gt;set_cachesize(0, CACHESIZE, 0);
        (void)dbenv-&gt;set_flags(DB_TXN_NOSYNC, 1);

        /*
         * Configure the local address. This is the local hostname and 
         * port that this replication environment will use to receive 
         * incoming replication messages. Note that this can be 
         * performed only once for the replication environment. 
         * It is required.
         */
       dbenv-&gt;repmgr_set_local_site(listen_host, listen_port, 0);

       /*
        * Set this replication environment's priority. This is used 
        * for elections.
        *
        * Set this number to a positive integer, or 0 if you do not want
        * this site to be able to become a master.
        */
        dbenv-&gt;rep_set_priority(100);

        /*
         * Add a site to the list of replication environments known to 
         * this application. 
         */
         dbenv-&gt;repmgr_add_remote_site(dbenv, other_host, other_port, 
                                       NULL, 0);

        /*
         * Identify the number of sites in the replication group. This is
         * necessary so that elections and permanent message handling 
         * can be performed correctly.
         */
         dbenv-&gt;rep_set_nsites(2);

        /* Open the environment handle. Note that we add DB_THREAD and
         * DB_INIT_REP to the list of flags. These are required.
         */
         dbenv-&gt;open(home, DB_CREATE | DB_RECOVER |
                           DB_INIT_LOCK | DB_INIT_LOG |
                           DB_INIT_MPOOL | DB_INIT_TXN  |
                           DB_THREAD | DB_INIT_REP,
                           0);

       /* 
        * Start the replication manager such that it uses 3 
        * threads. 
        */
         dbenv-&gt;repmgr_start(3, DB_REP_ELECTION);

       /* Sleep to give ourselves time to find a master */
        sleep(5);

       /*
        **********************************************************
        *** All other application code goes here, including  *****
        *** database opens                                   *****
        **********************************************************
        */


    } catch (DbException &amp;de) {
        /* Error handling goes here */
    }

    /* Close out the application here.
    try {
        /* 
         * Make sure all your database handles are closed 
         *  (omitted from this example). 
         */

        /* Close the environment */
        if (dbenv != NULL)
            (void)dbenv-&gt;close(dbenv, 0);

    } catch (DbException &amp;de) {
        /* Error handling goes here */
    }

    /* All done */ </pre>
        <div class="sect2" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h3 class="title"><a id="election_flags"></a>Managing Election Policies</h3>
              </div>
            </div>
          </div>
          <p>
                Before continuing, it is worth taking a look at the

                <span>
                startup election flags accepted by 
                
                <span><code class="methodname">DbEnv::repgmr_start()</code>.</span>
                These flags control how your replication application will
                behave when it first starts up.
                </span>

                
            </p>
          <p>
                    In the previous example, we specified 
                        <code class="literal">DB_REP_ELECTION</code>
                        
                    when we started replication. This causes the
                    application to try to find a master upon startup. If it
                    cannot, it calls for an election. In the event an
                    election is held, the environment receiving the most number of
                    votes will become the master.
            </p>
          <p>
                    There's some important points to make here:
            </p>
          <div class="itemizedlist">
            <ul type="disc">
              <li>
                <p>
                                    This 
                                    <span>flag</span> 
                                     
                                    only requires that other
                                    environments in the replication group
                                    participate in the vote. There is no
                                    requirement that
                                    <span class="emphasis"><em>all</em></span> such
                                    environments participate. In other
                                    words, if an environment
                                    starts up, it can call for an
                                    election, and select a master, even 
                                    if all other environment have not yet
                                    joined the  replication group.
                            </p>
              </li>
              <li>
                <p>
                                    It only requires a simple majority of
                                    participating environments to elect a master. The number of
                                    environments used to calculate the simple
                                    majority is based on the value set for

                                    
                                    <span><code class="methodname">DbEnv::rep_set_nsites()</code>.</span>
                                    
                                    
                                    
                                    This is always true of elections held using the Replication Manager.
                            </p>
              </li>
              <li>
                <p>
                                    As always, the environment participating in the election with the most
                                    up-to-date log files is selected as
                                    master. If an environment with more recent log files
                                    has not yet joined the replication
                                    group, it may not become the master.
                            </p>
              </li>
            </ul>
          </div>
          <p>
                    Any one of these points may be enough to cause a
                    less-than-optimum environment to be selected as master.
                    Therefore, to give you a better degree of control over
                    which environment becomes a master at application startup,
                    the Replication Manager offers the following start-up
                                    <span>flags:</span> 
                                     
            </p>
          <div class="informaltable">
            <table border="1" width="80%">
              <colgroup>
                <col />
                <col />
              </colgroup>
              <thead>
                <tr>
                  <th>Flag</th>
                  <th>Description</th>
                </tr>
              </thead>
              <tbody>
                <tr>
                  <td>
                    <code class="literal">DB_REP_MASTER</code>
                  </td>
                  <td>
                   <p>
                   The application starts up and declares the environment to be a master
                   without calling for an election. It is an error for more
                   than one environment to start up using this flag, or for
                   an environment
                   to use this flag when a master already exists. 
                   </p>
                   <p>
                           Note that no replication group should
                           <span class="emphasis"><em>ever</em></span> operate with more than
                           one master.
                   </p>
                   <p>
                           In the event that a environment attempts to become a
                           master when a master already exists, the
                           replication code will resolve the problem by
                           holding an election. Note, however, that there
                           is always a possibility of data loss in the face
                           of duplicate masters, because once a master is
                           selected, the environment that loses the election will
                           have to roll back any transactions committed
                           until it is in sync with the "real" master.
                   </p>

                  </td>
                </tr>
                <tr>
                  <td>
                    <code class="literal">DB_REP_CLIENT</code>
                  </td>
                  <td>
                          <p>
                                  The application starts up and declares
                                  the environment to be a replica without calling for
                                  an election. Note that the environment
                                  can still become a master if a subsequent
                                  application starts up, calls for an
                                  election, and this environment is elected
                                  master.
                          </p>
                  </td>
                </tr>
                <tr>
                  <td>
                    <code class="literal">DB_REP_ELECTION</code>
                  </td>
                  <td>
                          <p>
                          As described above, the application starts up,
                          looks for a master, and if one is not found calls
                          for an election.
                          </p>
                  </td>
                </tr>
              </tbody>
            </table>
          </div>
        </div>
        <div class="sect2" lang="en" xml:lang="en">
          <div class="titlepage">
            <div>
              <div>
                <h3 class="title"><a id="thread_count"></a>Selecting the Number of Threads</h3>
              </div>
            </div>
          </div>
          <p>
                    Under the hood, the Replication Manager is threaded and you can
                    control the number of threads used to process messages received from
                    other replicas. The threads that the Replication Manager uses are:
            </p>
          <div class="itemizedlist">
            <ul type="disc">
              <li>
                <p>
                                    Incoming message thread. This thread
                                    receives messages from the site's
                                    socket and passes those messages to
                                    message processing threads (see below)
                                    for handling.
                            </p>
              </li>
              <li>
                <p>
                                    Outgoing message thread. Outgoing
                                    messages are sent from whatever thread
                                    performed a write to the database(s).
                                    That is, the thread that called, for
                                    example, 
                                    
                                    <code class="methodname">Db::put()</code>
                                    
                                    is the thread that writes replication messages
                                    about that fact to the socket. 
                            </p>
                <p>
                                    Note that if this write activity would
                                    cause the thread to be blocked due to
                                    some condition on the socket, the Replication Manager
                                    will hand the outgoing message to the
                                    incoming message thread, and it will
                                    then write the message to the socket.
                                    This prevents your database write
                                    threads from blocking due to abnormal
                                    network I/O conditions.
                            </p>
              </li>
              <li>
                <p>
                                    Message processing threads are
                                    responsible for parsing and then
                                    responding to incoming replication
                                    messages. Typically, a response will
                                    include write activity to your
                                    database(s), so these threads can be
                                    busy performing disk I/O.
                            </p>
              </li>
            </ul>
          </div>
          <p>
                    Of these threads, the only ones that you have any
                    configuration control over are the message processing
                    threads. In this case, you can determine how many
                    of these threads you want to run.
            </p>
          <p>
                    It is always a bit of an art to decide on a thread count,
                    but the short answer is you probably do not need more
                    than three threads here, and it is likely that one will
                    suffice. That said, the best thing to do is set your
                    thread count to a fairly low number and then increase
                    it if it appears that your application will benefit
                    from the additional threads.
            </p>
        </div>
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